According to the prior art as illustrated in FIG. 1, a signal node 101 of a charge-storage element 100 is connected directly to a reset circuit 102 which normally comprises a single switch 103. In the phase in which the signal node is intended to be set to an initial value, the connection of the reset circuit 102 is connected with a low impedance to a voltage potential 104, by closing the switch 103.
In the other phase, the holding phase, the connection of the reset circuit is connected with a high impedance to a voltage potential 104 by opening the switch 103. During the holding phase, the voltage potential 104 can be varied as required. The impedance of the reset circuit 102 must be sufficiently high during the holding phase to avoid the voltage potential 104 reacting with the signal node 101.
The switch 103 can normally be provided by means of a transistor. One disadvantageous characteristic of a transistor is, however, that currents, so-called off-state currents or leakage currents, still flow into and out of the transistor connections even in the switched-off state.
If the switch 103 in the reset circuit 102 is formed by a transistor, the impedance of the reset circuit 102 will in some circumstances not be sufficiently high owing to the unavoidable off-state and leakage currents in the holding phase.
U.S. Pat. No. 5,854,498, which is incorporated herein by reference, describes a pixel sensor, in which the 1/f noise is reduced and the voltage range is increased, and the cell size is reduced. The threshold voltage is essentially overcome by using a reset diode and, in contrast to a reset transistor, this cannot be significantly influenced by the 1/f noise.